Varactor diode is linearly doped
voltage control capacitor diode which is used in reversed bias.
Varactor diode Capacitance can be
change by changing reverse bias voltage which effects frequency and
in frequency modulation signal frequency is modulated according to
input signal voltage.
Varactor diode is placed in
Hartley's oscillator which control the frequency according to
Capacitance.
Oscillator frequency is
As voltage of input signal changes
Capacitance of varactor diode will change which will change the
signal frequency results in frequency modulation.